Chapter 5. The /proc File System

The /proc File System

In Linux there is an additional mechanism for the kernel and kernel modules to send information to processes --- the
/proc file system. Originally designed to allow easy access to information about
processes (hence the name), it is now used by every bit of the kernel which has something interesting to report, such as
/proc/modules which has the list of modules and /proc/meminfo which has memory usage
statistics.

The method to use the proc file system is very similar to the one used with device drivers --- you create a structure
with all the information needed for the /proc file, including pointers to any handler
functions (in our case there is only one, the one called when somebody attempts to read from the /proc file). Then, init_module registers the structure with the kernel and
cleanup_module unregisters it.

The reason we use proc_register_dynamic[1] is because we don't want to determine the inode number
used for our file in advance, but to allow the kernel to determine it to prevent clashes. Normal file systems are located on a
disk, rather than just in memory (which is where /proc is), and in that case the inode
number is a pointer to a disk location where the file's index-node (inode for short) is located. The inode contains
information about the file, for example the file's permissions, together with a pointer to the disk location or locations
where the file's data can be found.

Because we don't get called when the file is opened or closed, there's no where for us to put
MOD_INC_USE_COUNT and MOD_DEC_USE_COUNT in this module, and if the file is opened and
then the module is removed, there's no way to avoid the consequences. In the next chapter we'll see a harder to implement, but
more flexible, way of dealing with /proc files which will allow us to protect against
this problem as well.

Example 5-1. procfs.c

/* procfs.c - create a "file" in /proc
*
* Copyright (C) 2001 by Peter Jay Salzman
*
* 08/02/2006 - Updated by Rodrigo Rubira Branco <rodrigo@kernelhacking.com>
*/
/* Kernel Programming */
#define MODULE
#define LINUX
#define __KERNEL__
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif
/* Necessary because we use the proc fs */
#include <linux/proc_fs.h>
/* In 2.2.3 /usr/include/linux/version.h includes a
* macro for this, but 2.0.35 doesn't - so I add it
* here if necessary. */
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif
/* Put data into the proc fs file.
Arguments
=========
1. The buffer where the data is to be inserted, if
you decide to use it.
2. A pointer to a pointer to characters. This is
useful if you don't want to use the buffer
allocated by the kernel.
3. The current position in the file.
4. The size of the buffer in the first argument.
5. Zero (for future use?).
Usage and Return Value
======================
If you use your own buffer, like I do, put its
location in the second argument and return the
number of bytes used in the buffer.
A return value of zero means you have no further
information at this time (end of file). A negative
return value is an error condition.
For More Information
====================
The way I discovered what to do with this function
wasn't by reading documentation, but by reading the
code which used it. I just looked to see what uses
the get_info field of proc_dir_entry struct (I used a
combination of find and grep, if you're interested),
and I saw that it is used in <kernel source
directory>/fs/proc/array.c.
If something is unknown about the kernel, this is
usually the way to go. In Linux we have the great
advantage of having the kernel source code for
free - use it.
*/
#if LINUX_VERSION_CODE > KERNEL_VERSION(2,4,0)
int procfile_read(char *buffer,
char **buffer_location, off_t offset,
int buffer_length, int *eof, void *data)
#else
int procfile_read(char *buffer,
char **buffer_location,
off_t offset,
int buffer_length,
int zero)
#endif
{
int len; /* The number of bytes actually used */
/* This is static so it will still be in memory
* when we leave this function */
static char my_buffer[80];
static int count = 1;
/* We give all of our information in one go, so if the
* user asks us if we have more information the
* answer should always be no.
*
* This is important because the standard read
* function from the library would continue to issue
* the read system call until the kernel replies
* that it has no more information, or until its
* buffer is filled.
*/
if (offset > 0)
return 0;
/* Fill the buffer and get its length */
len = sprintf(my_buffer,
"For the %d%s time, go away!\n", count,
(count % 100 > 10 && count % 100 < 14) ? "th" :
(count % 10 == 1) ? "st" :
(count % 10 == 2) ? "nd" :
(count % 10 == 3) ? "rd" : "th" );
count++;
/* Tell the function which called us where the
* buffer is */
*buffer_location = my_buffer;
/* Return the length */
return len;
}
#if LINUX_VERSION_CODE > KERNEL_VERSION(2,4,0)
struct proc_dir_entry *Our_Proc_File;
#else
struct proc_dir_entry Our_Proc_File =
{
0, /* Inode number - ignore, it will be filled by
* proc_register[_dynamic] */
4, /* Length of the file name */
"test", /* The file name */
S_IFREG | S_IRUGO, /* File mode - this is a regular
* file which can be read by its
* owner, its group, and everybody
* else */
1, /* Number of links (directories where the
* file is referenced) */
0, 0, /* The uid and gid for the file - we give it
* to root */
80, /* The size of the file reported by ls. */
NULL, /* functions which can be done on the inode
* (linking, removing, etc.) - we don't
* support any. */
(struct file_operations *) procfile_read, /* The read function for this file,
* the function called when somebody
* tries to read something from it. */
NULL /* We could have here a function to fill the
* file's inode, to enable us to play with
* permissions, ownership, etc. */
};
#endif
/* Initialize the module - register the proc file */
int init_module()
{
/* Success if proc_register[_dynamic] is a success,
* failure otherwise. */
#if LINUX_VERSION_CODE > KERNEL_VERSION(2,2,0)
/* In version 2.2, proc_register assign a dynamic
* inode number automatically if it is zero in the
* structure , so there's no more need for
* proc_register_dynamic
*/
#if LINUX_VERSION_CODE > KERNEL_VERSION(2,4,0)
Our_Proc_File=create_proc_read_entry("test", 0444, NULL, procfile_read, NULL);
if ( Our_Proc_File == NULL )
return -ENOMEM;
else
return 0;
#else
return proc_register(&proc_root, &Our_Proc_File);
#endif
#else
return proc_register_dynamic(&proc_root, &Our_Proc_File);
#endif
/* proc_root is the root directory for the proc
* fs (/proc). This is where we want our file to be
* located.
*/
}
/* Cleanup - unregister our file from /proc */
void cleanup_module()
{
#if LINUX_VERSION_CODE > KERNEL_VERSION(2,4,0)
remove_proc_entry("test", NULL);
#else
proc_unregister(&proc_root, Our_Proc_File.low_ino);
#endif
}
MODULE_LICENSE("GPL");